If we take a
series of images in time, and there are moving objects in the
scene, or perhaps the camera is itself on some moving vehicle,
useful information about what the image contains can be obtained by
analysing and understanding the difference between images caused
by the motion.

For example, given an image of
a moving car, deciding which pixels in the image represent motion
can help to decide which pixels belong to the car, and which
to the static background.

Studying the motion in detail, we can answer such
questions as

How many moving objects there are?

Which directions
they are moving in?

Whether they are undergoing linear or
rotational motion?

How fast they are moving?

From an image sequence, we calculate a new function called
the optical flow. For every pixel,